The purpose of this study is to measure and compare gamma-aminobutyric acid (GABA) levels in the occipital cortex of a group of healthy smoking and non-smoking women and men ages 18-50. We will recruit women with regular menstrual cycles so that we can assess premenstrual impact of smoking cessation in that population and compare GABA level concentrations across all groups. Although there are several note-worthy differences between male and females in regard to smoking behavior, ultimately none are as worrisome as the disparity in ability to quit smoking. While multiple explanations for why women are less successful in their abstinence attempts have been proffered, the observation that women are more likely to experience emergence of depressive symptoms during smoking cessation, a known risk factor for relapse, may be the most important contributor to this sex-specific recidivism. Several lines of evidence suggest that nicotine modulation of GABA may play an important role in this interplay between sex, depression, and smoking recidivism.